To obtain hydrocarbons such as oil and gas, wellbores or boreholes are drilled from one or more surface locations into hydrocarbon-bearing subterranean geological strata or formations (also referred to in the industry as the reservoirs). A large proportion of the current drilling activity involves drilling highly deviated and/or substantially horizontal wellbores extending through the reservoir.
To develop an oil and gas field, especially an offshore field, multiple wellbores are drilled from an offshore rig or platform stationed at a fixed location. A template is placed on the earth's surface that defines the location and size of each wellbore to be drilled. The various wellbores are then drilled along their respective predetermined paths.
Whether for onshore drilling or offshore drilling of multiple wellbores from a common location, each wellbore is drilled to a predetermined depth in the earth's surface. Frequently, ten to twenty offshore wellbores are drilled from an offshore rig stationed at a single location. Each such wellbore is drilled to a respective predetermined vertical depth and then deviated to reach a desired subterranean formation.
The above-described wellbore construction requires drilling a top or surface portion of the wellbore, with a large diameter to accommodate the casing, and then drilling the production or pilot wellbore, which is relatively smaller in diameter. There are many other situations in the well-drilling business where it is required to drill a hole a size larger than the prior hole drift. Many tools have been developed over the past thirty years to drill the oversized hole sections. These tools include conventional under-reamers, bi-center bit and ream-while-drilling tools. Until most recently these tools have not been used extensively in directional, drilling-while-reaming applications primarily due to torque requirements of the downhole motor.
If the wellbore is vertical, few problems exist in running the two drilling operations (drilling the pilot hole and opening the hole to a larger diameter) at the same time. A pilot bit drills the pilot hole and an apparatus such as an under-reamer, which is positioned uphole from the pilot bit, follows along the same line as the pilot bit and opens the pilot hole to the desired diameter to accommodate the casing. New drilling methods, however, frequently require that the wellbore be deviated--drilled at an angle to the vertical axis. This deviation causes problems for the under-reamer since it no longer follows vertically into the wellbore after the drill bit. Using current apparatus, the under-reamer is now operating along an axis that does not correspond to the axis of the pilot bit.
An important aspect of drilling a deviated or horizontal wellbore is to drill it along a predetermined wellpath. During drilling of the wellbore, it is important to accurately determine the true location of the pilot bit relative to a reference point so as to continuously maintain the pilot bit along the desired wellpath. The current drill strings usually include a large number of sensors to provide information about the pilot bit location, formation parameters, borehole parameters and the tool condition and a relatively low data transmission telemetry, such as the mud-pulse telemetry. In such systems, the pilot bit location data is transmitted to the surface periodically and used to send directional instructions to keep the pilot bit on course.
The next problem to solve is to provide consistent directional control and stability for the under-reamer operations. Commonly used rotating stabilizers are satisfactory for pilot hole drilling and for vertical drilling of a pilot hole with simultaneous under-reamer drilling to open the hole. They may not be effective, however, in providing the required stability when the under-reamer is operated simultaneously with a pilot bit in drilling deviated wellbores. It is important to provide a point of stability close to the under-reamer to prevent wobbling of the under-reamer while it is drilling a larger borehole behind the pilot hole. Additionally, the current under-reamer operations make the downhole motor operate less efficiently due to additional stress caused by the under-reamer drilling along an axis that is not in line with the axis of the pilot bit.
The present invention addresses the above-described problems with the prior art methods for drilling. It uses an under-reamer/bi-center bit or ream-while -drilling tool run below a bent housing motor. In one embodiment of the invention, a non-rotating stabilizer blade is placed between the reamer arms and the pilot bit. The non-rotating stabilizer is an integral part of the reamer body to minimize the distance between the stabilizer blades and the reamer arms. The distance between the reamer arms and the stabilizer blades minimizes side loads on the reamer arms. Because the stabilizer is non-rotational, the torque output of the bent housing motor is reduced which is an important factor in drilling larger hole sizes. However, a non-rotating stabilizer is more complex and less rugged than a rotating stabilizer.
In an alternate embodiment of the invention, the stabilizer is located between the under-reamer and the drill bit but is fixed with respect to the drill bit. In such an arrangement, provision is made in the stabilizer for passages to allow return flow of drilling mud to the surface. In yet another embodiment of the invention, a stabilizer that adjustably engages the borehole is located above under-reamer.